The use of elementary detectors, and particularly photodiodes, to detect electromagnetic radiation is known. Indeed, incident electromagnetic radiation on a photodiode in fact creates electrical charges therein. These charges are usually collected and accumulated in an integrating capacitance so that they can subsequently be read. This device is improved by allowing the integration and read phases to overlap in time. To this end, a reading capacitance and a transfer switch are usually used.
FIG. 1 shows schematically, under the general reference 10, a prior art device of this kind for reading the electrical charges produced in a photodiode 12 belonging to a matrix of photodiodes in a detector of electromagnetic radiation, such as infrared radiation, visible radiation or ultra-violet radiation for example. This device allows the charges to be integrated and read simultaneously.
Said read device 10, associated with each photodiode 12 of the detector, comprises:                a first so-called “integrating” capacitor 14;        a second so-called “read” capacitor 16;        an injection circuit 18 the function of which, when it is enabled, is to bias the photodiode 12 and to transfer the electrical charges produced by it to the integrating capacitor 14. Another function of the injection circuit 18 is to discharge the integrating capacitor 14;        a switch 20 the function of which, when it is in its closed state, is to transfer the electrical charges stored in the integrating capacitor 14 to the read capacitor 16;        a read circuit 22 the function of which is to read the voltage at the terminals of the read capacitor 16 and to transmit by multiplexing the voltage measurement to the outside of the read device 10, for example on a column bus of the detector, the photodiode matrix being read by scanning, as is known per se. Another function of the read circuit 22 is to discharge the read capacitor 16; and        a control circuit 24 controlling the injection circuit 18, the switch 20 and the read circuit 22. The control circuit 24 usually also controls the other read circuits associated with the detector photodiodes, in order to synchronize all the control signals, as is known per se.        
More particularly, the control circuit 24 controls:                by means of a signal “INJ”, the enabling and disabling of the bias of the photodiode 12 and of the transfer of the charges produced thereby, implemented by the injection circuit 18;        by means of a binary signal “RINT”, the discharge of the integrating capacitor 14, implemented by the injection circuit 18;        by means of a binary signal “TFX”, the status of the switch 20 for transferring the charges from the integrating capacitor 14 to the read capacitor 16;        by means of a binary signal “RLECT”, the discharge of the read capacitor 16, implemented by the read circuit 22; and        by means of a binary signal “LECT”, the reading of the voltage at the terminals of the read capacitor 16 and the transmission of the read voltage to the outside of the device 10, implemented by the read circuit 22.        
In FIG. 2, control signal timing diagrams for the control circuit 24 show a method for reading the electrical charges produced by the photodiode 12 as implemented by the read device 10 according to the prior art.
At 30, the signal “INJ” is switched into its high state. The charges produced in the photodiode 12 under the effect of an incident radiation are then stored, via the injection circuit 18, in the integrating capacitor 14, the latter being previously reset by the signal “RINT”. After a predetermined period Tint, the signal “INJ” is switched, at 32, into its low state, the storage of the charges produced by the photodiode 12 in the integrating capacitor 14 then being stopped.
Once the storage of the charges in the integrating capacitor 14 has stopped (“INJ” in its low state), the signal “TFX” is switched, at 34, into its high state in order to transfer, via the switch 20, the charges stored in the integrating capacitor 14 to the read capacitor 16, the latter being previously reset by the signal “RLEC”. Once this, almost instantaneous, transfer is completed, the signal “TFX” is switched, at 36, into its low state. The read signal “LECT” is then switched, at 38, into its high state in order to read the voltage at the terminals of the read capacitor 16 and to transfer the read voltage to the outside of the device 10.
The integrating 14 and read 16 capacitors are then discharged by switching the signals “RINT” and “RLEC” into their high state at 40 after the end of integration and 42 after the end of reading respectively. A new integration and read cycle can then start after a new switching of the signal “RINT” at 44 and the signal “RLEC” at 43 respectively.
It will be noted that the reading implemented by the read circuit 22 comes to an end while the storage of the charges in the integrating capacitor is effective (signal “INJ” in its high state) and the transfer of the charges between the integrating and read capacitors 14, 16 is disabled (signal “TFX” in its low state). Indeed, as is known per se, the voltage read by the read circuit 22 is delivered in multiplexed mode on a column bus of the detector, the matrix of photodiodes of the detector being read line by line. In fact, reading and delivering a voltage in a matrix read by scanning takes some time. In order not to suspend over this period of time the storage of the charges in the integrating capacitor 14 on account of a direct reading of the voltage at the terminals thereof, the read capacitor 16 is provided. Transferring the charges produced by the photodiode 12 into the read capacitor 16 thus allows the voltage to be read at the terminals thereof, while a new cycle of accumulating the charges produced by the photodiode 12 in the integrating capacitor 14 has started.
A first limitation on the operation of the read device so described stems from the limited size of the integrating capacitor 14. Indeed, the surface usually allocated to the read device 10 is limited for reasons of compactness. The total electrical charge that can be stored in the read capacitor 14, which depends on the latter's size, is therefore limited. The detector dynamic is thus also limited. By way of example of said limited dynamic, the integrating capacitors associated with the photodiode matrix thereof rapidly saturate under the effect of intense radiation.
A second limitation relates to the read noise. Indeed, below a certain charge stored in the read capacitor 16, a charge which is dependent on the value of the capacitance thereof, voltage reading at the terminals of the read capacitor 16 is marred by a significant noise relative to the voltage read.
The read device, in its conventional operation, therefore sees its dynamic limited, both in the high charges (limited total storable charge) and in the low charges (low charge reading with added noise effects).
To overcome these drawbacks, a proposal has been made in the document “Performance of BF focal plane array 320×256 InSb detector” by O. Nesher, Semi Conductor Devices, proc. of SPIE, vol. 4820, page 699, for a double integration of the charges produced by the photodiode 12.
According to this technique, shown in FIG. 3, two integrations of different durations are performed concomitantly, an injection phase “A” with an integration time Tinta, shorter than the integration time Tint precedes an injection phase “B1” with the integration time Tint1. An injection phase “B2”, with an injection time Tint2, may precede the injection phase “A” in order to improve the temporal coherence, the read value “B” then corresponding to the total amount of the charges built up during Tint1 and Tint2.
An increased dynamic is thus obtained, since the total “storable” charge is multiplied virtually by the coefficient
                    Tint        ⁢                                  ⁢        1            +              Tint        ⁢                                  ⁢        2                    Tint      ⁢                          ⁢      a        .In addition, two voltage measurements are obtained, the first corresponding to a short period of integrating the charge in the integrating capacitor 14, and the second corresponding to a long period of integrating in the capacitor 14. The first measurement thus comprises information about the highest part of the signal dynamic and the second measurement comprises information about the lowest part of the signal. The two measurements can be combined to obtain a large dynamic measurement.
Temporal coherence is simulated by framing the injection phase “A” with the two injection phases “B1” and “B2”. It will be noted that this technique operates satisfactorily for gradually changing incident fluxes. However, a pulsed incident flux may not be acquired simultaneously by the phases “A” and “B1+B2”. For an incident flux of this kind, temporal coherence is not obtained.
Furthermore, the choice of a smaller capacity capacitor for phases “B1” and “B2” is made possible as an addition to phase “A”, which means that the overall noise level of the signal obtained can be reduced. However, successive samplings of phases “B1” and “B2” help to increase reader noise.
In addition, as is pointed out in the aforementioned document, a specific read device is used. To fulfill all the functionalities listed in this document, said device is likely to comprise an additional capacitor, which is disadvantageous in so far as the read device is usually integrated into a small surface, for example into a small pitch matrix detector.